Redox flow battery with electrolyte balancing and compatibility enabling features

ABSTRACT

A redox flow battery includes first and second cells. Each cell has electrodes and a separator layer arranged between the electrodes. A first circulation loop is fluidly connected with the first electrode of the first cell. A polysulfide electrolyte solution has a pH 11.5 or greater and is contained in the first recirculation loop. A second circulation loop is fluidly connected with the second electrode of the second cell. An iron electrolyte solution has a pH 3 or less and is contained in the second circulation loop. A third circulation loop is fluidly connected with the second electrode of the first cell and the first electrode of the second cell. An intermediator electrolyte solution is contained in the third circulation loop. The cells are operable to undergo reversible reactions to store input electrical energy upon charging and discharge the stored electrical energy upon discharging.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure is a divisional of U.S. patent application Ser.No. 16/052,727 filed Aug. 2, 2018.

BACKGROUND

Flow batteries, also known as redox flow batteries or redox flow cells,are designed to convert electrical energy into chemical energy that canbe stored and later released when there is demand. As an example, a flowbattery may be used with a renewable energy system, such as awind-powered system, to store energy that exceeds consumer demand andlater release that energy when there is greater demand.

A typical flow battery includes a redox flow cell that has a negativeelectrode and a positive electrode separated by an electrolyte layer,which may include a separator, such as an ion-exchange membrane. Anegative fluid electrolyte (sometimes referred to as the anolyte) isdelivered to the negative electrode and a positive fluid electrolyte(sometimes referred to as the catholyte) is delivered to the positiveelectrode to drive reversible redox reactions between redox pairs. Uponcharging, the electrical energy supplied causes a chemical reductionreaction in one electrolyte and an oxidation reaction in the otherelectrolyte. The separator prevents the electrolytes from freely andrapidly mixing but permits selected ions to pass through to complete theredox reactions. Upon discharge, the chemical energy contained in theliquid electrolytes is released in the reverse reactions and electricalenergy can be drawn from the electrodes.

SUMMARY

A method for a redox flow battery according to an example of the presentdisclosure includes using first and second cells of a redox flow batteryto store input electrical energy upon charging and discharge the storedelectrical energy upon discharging. Each cell has a separator layerarranged between first and second electrodes. A polysulfide electrolytesolution of pH 11.5 or greater is circulated through a first circulationloop in fluid connection with the first electrode of the first cell, andan iron electrolyte solution of pH 3 or less is circulated through asecond circulation loop in fluid connection with the second electrode ofthe second cell, and an intermediator electrolyte solution is circulatedthrough a third circulation loop in fluid connection with the secondelectrode of the first cell and the first electrode of the second cell.Sulfur from the polysulfide electrolyte solution in the first electrodeof the first cell permeates through the ion-exchange layer of the firstcell and precipitates as a solid sulfide product in the second electrodeand iron from the iron electrolyte solution in the second electrode ofthe second cell permeates through the ion-exchange layer of the secondcell and precipitates as solid iron product. The intermediatorelectrolyte solution is emptied from either the second electrode of thefirst cell or the first electrode of the second cell, either the solidsulfide product is recovered to the polysulfide electrolyte solution orthe solid iron product is recovered to the iron electrolyte solution by,respectively, circulating at least a portion of the polysulfideelectrolyte solution from the first circulation loop through the secondelectrode to dissolve, and thereby remove, the solid sulfide productfrom the second electrode of the first cell, and then transferring thepolysulfide electrolyte solution with the dissolved solid sulfideproduct back in to the first loop, or circulating at least a portion ofthe iron electrolyte solution from the second circulation loop throughthe first electrode to dissolve, and thereby remove, the solid ironproduct from the first electrode of the second cell, and thentransferring the iron electrolyte solution with the dissolved solid ironproduct back in to the second loop.

A further embodiment of any of the foregoing embodiments includesmaintaining the intermediator electrolyte solution at a pH 12 or greaterso that the iron precipitates upon permeation through the ion-exchangelayer from the second electrode of the second cell into the firstelectrode of the second cell.

A further embodiment of any of the foregoing embodiments includesmaintaining the intermediator electrolyte solution at a pH 12 or greaterso that the sulfur precipitates upon permeation through the ion-exchangelayer from the first electrode of the first cell into the secondelectrode of the first cell.

A method for a redox flow battery according to an example of the presentdisclosure includes using first and second cells of a redox flow batteryto store input electrical energy upon charging and discharge the storedelectrical energy upon discharging. Each cell has a separator layerarranged between first and second electrodes. A polysulfide electrolytesolution of pH 11.5 or greater is circulated through a first circulationloop in fluid connection with the first electrode of the first cell, aniron electrolyte solution of pH 3 or less is circulated through a secondcirculation loop in fluid connection with the second electrode of thesecond cell, and an intermediator electrolyte solution is circulatedthrough a third circulation loop in fluid connection with the secondelectrode of the first cell and the first electrode of the second cell.A third cell is used to electrolyze either the polysulfide electrolytesolution to produce hydrogen gas or the iron electrolyte solution toproduce oxygen gas. The pH of the polysulfide electrolyte solution ismaintained to be pH 11.5 or greater or the pH of the iron electrolytesolution is maintained to be pH 3 or less by, respectively, introducingthe oxygen gas into the polysulfide electrolyte solution to adjust thepH of the polysulfide electrolyte solution, or introducing the hydrogengas into the iron electrolyte solution to adjust the pH of the ironelectrolyte solution.

In a further embodiment of any of the foregoing embodiments, theintroducing of the oxygen gas includes sparging the oxygen gas throughthe polysulfide electrolyte solution.

In a further embodiment of any of the foregoing embodiments, theintroducing of the hydrogen gas includes sparging the hydrogen gasthrough the iron electrolyte solution.

A method for a redox flow battery according to an example of the presentdisclosure includes using a cell of a redox flow battery to store inputelectrical energy upon charging and discharge the stored electricalenergy upon discharging. The cell has a separator layer arranged betweenfirst and second electrodes. A polysulfide electrolyte solution of pH11.5 or greater is circulated through a first circulation loop in fluidconnection with the first electrode of the cell, and a manganateelectrolyte solution is circulated through a second circulation loop influid connection with the second electrode of the cell. Sulfur from thepolysulfide electrolyte solution in the first electrode permeatesthrough the ion-exchange layer and precipitates as a solid sulfideproduct in the second electrode and manganese from the manganateelectrolyte solution in the permeates through the ion-exchange layer ofthe second cell and precipitates as solid iron product in the firstelectrode. Either the solid sulfide product is recovered to thepolysulfide electrolyte solution or the solid manganese product isrecovered to the manganate electrolyte solution by, respectively,circulating at least a portion of the polysulfide electrolyte solutionfrom the first circulation loop through the second electrode todissolve, and thereby remove, the solid sulfide product from the secondelectrode, and then transferring the polysulfide electrolyte solutionwith the dissolved solid sulfide product back in to the first loop, orcirculating at least a portion of the manganate electrolyte solutionfrom the second circulation loop through the first electrode todissolve, and thereby remove, the solid manganese product from the firstelectrode, and then transferring the manganese electrolyte solution withthe dissolved solid iron product back in to the second loop.

A further embodiment of any of the foregoing embodiments includespassing the polysulfide electrolyte solution with the dissolved solidsulfide product in a first direction through a bi-directional filter andpassing the manganate electrolyte solution with the dissolved solidmanganese product in a second, opposite direction through thebi-directional filter.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example redox flow battery.

FIG. 2 illustrates the redox flow battery with flush lines.

FIG. 3 illustrates a method for recovering sulfur, iron, or manganese ina redox flow battery.

FIG. 4 illustrates a bypass line and a third cell in the first loop ofthe redox flow battery.

FIG. 5 illustrates a bypass line and a third cell in the second loop ofthe redox flow battery.

FIG. 6 illustrates an example method of maintaining pH in thepolysulfide electrolyte solution or the iron electrolyte solution of theflow battery.

FIG. 7 illustrates another example redox flow battery and method inwhich sulfur and manganese are used as active species and precipitate ofeach is recovered using a recovery strategy method.

DETAILED DESCRIPTION

Redox flow batteries (“RFB”) utilize electrochemically active speciesthat include ions of elements that have multiple, reversible oxidationstates in a selected liquid solution. Example species may includetransition metals, such as vanadium, iron, manganese, chromium, zinc, ormolybdenum, or other elements such as sulfur, cerium, lead, tin,titanium, germanium, bromine, or chlorine. Although these species havebeen used, not all of them are compatible for use together. Forinstance, over time, there is mixing of species due to cross-over of thespecies through the separator. If incompatible, the cross-over speciesmay react to precipitate as an insoluble solid or to generate gases thatescape to the surroundings. Insoluble solids may block flow and debitperformance. Gaseous losses may pose a health concern and may reducespecies availability for proper functionality.

Two species that are attractive for use in RFBs due to low cost are ironand sulfur. However, iron and sulfur electrolytes, such as iron inhydrochloric acid and sulfur in sodium hydroxide, are highlyincompatible. Iron that crosses-over into the basic pH sulfurelectrolyte reacts to form insoluble iron oxide (Fe₂O₃) and sulfur thatcrosses-over into the acidic pH iron electrolyte reacts to form gaseoushydrogen sulfide (H₂S). Over time, the loss of iron, clogging from theinsoluble iron oxide, and the loss of sulfur to hydrogen sulfide willrender the RFB inoperable or, at the least, reduce round-trip efficiencyto unfeasible levels for use as an RFB. As will be discussed below, thedisclosed RFB utilizes an intermediator electrolyte that mitigatesincompatibility and enables use of sulfide and iron electrolytes in theRFB.

FIG. 1 schematically illustrates an RFB 20. The RFB 20 includes a firstcell 22 and a second cell 24. The cells 22 and 24 are of nominallyidentical constriction. The cells 22 and 24 include, respectively, firstelectrodes 22 a and 24 a, second electrodes 22 b and 24 b, and separatorlayers 22 c and 24 c between the electrodes (layer 22 c betweenelectrodes 22 a and 22 b, and layer 24 c between electrodes 24 a and 24b). For example, the electrodes 22 a, 22 b, 24 a, and 24 b are porouscarbon structures, such as carbon paper or felt. The separator layersare ion-exchange membranes 22 c and 24 c, which permit selected ions topass through to complete the redox reactions while electricallyisolating the electrodes.

A first circulation loop 26 is fluidly connected with the firstelectrode 22 a of the first cell 22, and a second circulation loop 28 isfluidly connected with the second electrode 24 b of the second cell 24.As used herein, a “loop” refers to a continuous, closed circuit fluidpassage. The first circulation loop 26 and the second circulation loop28 may include respective electrolyte storage tanks 30 and 32. Apolysulfide electrolyte solution 34 is contained in the firstrecirculation loop 26 (e.g., in the tank 30), and an iron electrolytesolution 36 is contained in the second circulation loop 28 (i.e., in thetank 32). The polysulfide electrolyte solution 34 has a pH 11.5 orgreater, and the iron electrolyte solution has a pH 3 or less.

The polysulfide in the polysulfide electrolyte solution 34 generallyrefers to salts of sulfur in a basic pH solution. For example, the saltis sodium salt with the formula Na₂S_(x), where x is 2 to 5, in sodiumhydroxide. In one example, the polysulfide electrolyte solution 34 maybe 2M Na₂S_(x) in 1M sodium hydroxide. The iron in the iron electrolytesolution 36 generally refers to iron salts in an acidic solution. In oneexample, the iron electrolyte solution 36 may be 1M FeCl_(x) in 1M NaCland 0.3M HCl.

The RFB 20 further includes a third circulation loop 38 fluidlyconnected with the second electrode 22 b of the first cell 22 and thefirst electrode 24 a of the second cell 24. The third circulation loop38 may include an electrolyte storage tank 40. The third circulationloop 38 contains an intermediator electrolyte solution 42 (i.e., in thetank 42) that participates in reactions in both cells 22 and 24. Theintermediator electrolyte solution 42 has a pH 12 or greater. Forexample, the intermediator electrolyte solution 42 includes at least oneof quinoxaline, anthraquinone, or benzoquinone. In one example, theintermediator electrolyte solution 42 includes at least one of1,2-benzoquinone-3,5-disulfonic acid,4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (2,6-DBEAQ),1,2-DBEAQ, or 1,8-DBEAQ. Other functionalized hydroxylatedanthraquinones, e.g. 2,6-dihydroxyanthraquinone (2,6-DHAQ), may also beused. Other organic-based redox couples include molecules based onviologens, quinoxalines, or alloxazines. Organomettalic reactants mayalso be used, such as ferrocenes. In one example, the intermediatorelectrolyte solution is 0.4M NaFe(CN)₆ in 1M NaOH and 1M NaCl. Inanother example, the intermediator electrolyte solution is 0.5M2,6-DBEAQ in 0.5M NaOH and 0.5M NaCl.

The polysulfide electrolyte solution 34 circulates through the firstelectrode 22 a of the first cell 22 and the iron electrolyte solutioncirculates through the second electrode 24 b of the second cell 24. Theintermediator electrolyte solution 42 circulates through the secondelectrode 22 b of the first cell 22 and the first electrode 24 a of thesecond cell 24. The polysulfide electrolyte solution 34 and theintermediator electrolyte solution 42 in the first cell 22, and the ironelectrolyte solution 36 and the intermediator electrolyte solution 42 inthe second cell 24, are operable to undergo reversible reactions tostore input electrical energy upon charging and discharge the storedelectrical energy upon discharging. The electrical energy may betransmitted to and from the cells 22 and 24 through an electric circuitthat is electrically coupled with the electrodes 22 a, 22 b, 24 a, and24 b.

The following equations demonstrate example reactions in the first cell22, as well as the resulting standard electrode potential (E^(o)) versusStandard Hydrogen Electrode (SHE) and Open Cell Voltage (OCV) is definedherein as the difference of the standard electrode potentials of the twoelectrode reactions.2Na₂S₂↔Na₂S₄+2Na⁺+2e′;E^(o)=−0.45vs SHE[Fe(CN)₆]³⁻ +e′↔[Fe(CN)₆]⁴⁻;E^(o)=+0.36vs SHEOCV=0.81V

The following equations demonstrate example reactions in the second cell24, as well as the resulting standard electrode potential (E^(o)) versusStandard Hydrogen Electrode (SHE) and Open Cell Voltage (OCV).[Fe(CN)₆]⁴⁻↔[Fe(CN)₆]³⁻ +e′;E^(o)=+0.36vs SHE2FeCl₃+2Na⁺+2e′↔2FeCl₂+2NaCl;E^(o)=+0.771vs SHEOCV=0.41VThe net reaction is:2Na₂S₂+2FeCl₃→Na₂S₄+2FeCl₂+2NaClOCV=1.218V

As discussed above, polysulfide and iron electrolyte solutions aregenerally incompatible in RFBs. However, the intermediator electrolytesolution 42 in the RFB 20 mitigates the incompatibility and enables useof sulfide and iron electrolytes together. For instance, rather thansulfur crossing-over into an iron solution, the sulfur in the RFB 20crosses-over into the intermediator electrolyte solution 42. And ratherthan iron crossing-over into a sulfur solution, the iron in the RFB 20crosses-over into the intermediator electrolyte solution 42. Theintermediator electrolyte solution 42 is selected to produce moredesirable reactions with the sulfur and the iron such that the sulfurand iron can readily be recovered and returned to their respectivesolutions 34 and 36.

For example, when sulfur crosses-over from the first electrode 22 athrough the ion-exchange layer 22 c and into the intermediatorelectrolyte solution 42 at the second electrode 22 b, the sulfurprecipitates as a solid sulfur product. When iron crosses-over from thesecond electrode 24 b through the ion-exchange layer 24 c and into theintermediator electrolyte solution 42 at the first electrode 24 a, theiron precipitates as a solid iron product. The conditions for the sulfurand the iron to precipitate require that the pH of the intermediatorelectrolyte solution 42 be 12 or greater and that the standard electrodepotential be greater than −0.3V. At pH less than approximately 12 orstandard electrode potential less than approximately −0.3V, the sulfurmay react to form hydrogen sulfide gas and the iron may react to forminsoluble iron oxide. As will be discussed later below, the solid sulfurproduct and solid iron oxide product can readily be recovered andincorporated back into, respectively, the polysulfide electrolytesolution 34 and the iron electrolyte solution 36 to maintain RFBperformance.

In addition to a pH of 12 or greater and a standard electrode potentialof −0.3V or higher, the selected intermediator electrolyte solution 42has highly reversible kinetics between its reduction and oxidationreactions, has ionic function groups (e.g., OH⁻), and is a largemolecule to reduce cross-over of the intermediator. Solubility of theintermediator electrolyte solution 42 is not critical, since theintermediator electrolyte solution 42 maintains a state-of-charge ofapproximately 50% at all times and only a limited quantity of theintermediator electrolyte is required (i.e., the amount does notdetermine the total energy capacity of the battery).

FIG. 2 depicts a further example of the RFB 20. In this example, the RFB20 additionally includes one or more flush lines 44. The flush lines 44may be used to remove and recover the solid sulfur product and the solidiron product according to an electrolyte-takeover method (ETM) 50illustrated in FIG. 3 . The method 50 generally includes steps 52 and54. At step 52 the intermediator electrolyte solution 42 is emptied fromthe second electrode 22 b of the first cell and/or the first electrode24 a of the second cell 24. At step 54 the polysulfide electrolytesolution 34 is circulated via the flush line 44 through the secondelectrode 22 b of the first cell and/or the iron electrolyte solution 36is circulated via the flush line 44 through the first electrode 24 a ofthe second cell 24. The solid sulfide product is readily soluble in thepolysulfide electrolyte solution 34. The polysulfide electrolytesolution 34 thus dissolves and removes the solid sulfide product fromthe second electrode 22 b. Similarly, the solid iron product is readilysoluble in the iron electrolyte solution 36. The iron electrolytesolution 36 thus dissolves and removes the solid iron product from thefirst electrode 24 b.

Once the solid sulfur and/or iron products have been removed to adesired level, the polysulfide electrolyte solution 34 is thentransferred back into the first loop 26 and the iron electrolytesolution 36 is transferred back into the second loop 28. Theintermediator electrolyte solution 42 can then resume circulationthrough the cells 22 and 24 to charge or discharge the RFB 20.

FIGS. 4 and 5 depict further examples of the RFB 20. In these examples,the RFB 20 additionally includes a bypass line 56 and a third cell 58 inthe bypass line 56. In FIG. 4 , the bypass line 56 and the third cell 58are in the first loop 26, and in FIG. 5 the bypass line 56 and the thirdcell 58 are in the second loop 28. The bypass line 56 and the third cell58 may be used according to a method 60 in FIG. 6 to maintain the pH ofthe polysulfide electrolyte solution 34 to be pH 11.5 or greater or thepH of the iron electrolyte solution 36 to be pH 3 or less.

At step 62, the third cell 58 is used to electrolyze the polysulfideelectrolyte solution 34 (FIG. 4 ) or the iron electrolyte solution 36(FIG. 5 ). The third cell 58 is an electrolyzer cell that uses inputelectrical power to drive an electrolysis reaction of the polysulfideelectrolyte solution 34 to generate hydrogen gas or the iron electrolytesolution 36 to generate oxygen gas. The respective net reactions are asfollows.

(polysulfide electrolyte solution)2H₂O+2Na₂S₂→Na₂S₄+2NaOH+H₂(g)

(iron electrolyte solution)2H₂O+4FeCl₃→4FeCl₂+4HCl+O₂(g)

At step 64, the hydrogen can be introduced into the iron electrolytesolution 36 to adjust the pH of the iron electrolyte solution 36 and/orthe oxygen gas can be introduced into the polysulfide electrolytesolution 34 to adjust the pH of the polysulfide electrolyte solution 34.For instance, the introducing of the hydrogen gas involves sparging(bubbling) the hydrogen gas through iron electrolyte solution 36, suchas in the tank 32 as shown at 66 (FIG. 4 ). The hydrogen reacts with theiron electrolyte solution 36 to lower the pH. The introducing of theoxygen gas involves sparging (bubbling) the oxygen gas throughpolysulfide electrolyte solution 34, such as in the tank 30 as shown at68 (FIG. 5 ). The oxygen reacts with the polysulfide electrolytesolution 34 to increase the pH. In these regards, the third cell 58 maybe connected by a bleed line 70 to either the second loop 28 (FIG. 4 )or the first loop (FIG. 5 ).

Additional species that are attractive for use in RFBs due to low costare permanganate and sulfur. However, manganate species that cross-overinto the low potential sulfur electrolyte reduces to form insolublemanganate hydroxide Mn(OH)₂ and sulfur that crosses-over into themanganate electrolyte oxidizes to form solid sulfur metal. Over time,the loss of sulfur and manganate species, and clogging from theinsoluble sulfur and manganate species will render the RFB inoperableor, at the least, reduce round-trip efficiency to unfeasible levels foruse as an RFB. As will be discussed below, the disclosed RFB thatutilizes manganate and sulfur does not necessarily require anintermediator electrolyte; however, it may employ similar recoverystrategies as described above to mitigate the issues resulting fromcrossover in the RFB.

The desired reactions in the polysulfide and maganate RFB are:Negative: 2Na₂S₂↔Na₂S₄+2Na⁺+2e ⁻E^(o)=−0.447vs. SHEPositive: 2NaMnO₄+2Na⁺+2e ⁻↔2Na₂MnO₄E^(o)=+0.558vs. SHENet cell: 2Na₂S₂+2NaMnO₄↔Na₂S₄+2Na₂MnO₄OCV=1.01V

The fact that crossover results in solid deposits enables the S and Mnto be separated and returned to their original electrolyte. If solidsare deposited in the electrodes, the ETM method 50 described above (forthe Fe and S system) can be applied to dissolve and return the solidspecies to their original electrolyte. Furthermore, if these solids arein the membrane, then exposing both electrodes to the same electrolyteshould enable dissolution and recovery. This recovery mechanism isexpected to be quick (i.e., <1 hour) and, if needed, it can be conductedat elevated temperatures to accelerate the process. The end result isthat electrolyte balance is maintained.

To illustrate, FIG. 7 shows another example RFB 120. The RFB 120 in thisexample does not include a third circulation loop and only includes asingle, common cell 122 or stack of common cells. The cell 122 includesa first electrode 122 a, a second electrode 122 b, and separator layers122 c between the electrodes 122 a and 122 b. In this disclosure, likereference numerals designate like elements where appropriate andreference numerals with the addition of one-hundred or multiples thereofdesignate modified elements that are understood to incorporate the samefeatures and benefits of the corresponding elements.

The first circulation loop 26 is fluidly connected with the firstelectrode 122 a of the cell 122, and a second circulation loop 28 isfluidly connected with the second electrode 122 b of the cell 122. Thepolysulfide electrolyte solution 34 is contained in the firstrecirculation loop 26 (e.g., in the tank 30), and a manganateelectrolyte solution 136 is contained in the second circulation loop 28(i.e., in the tank 32).

In the ETP method 50 the polysulfide electrolyte solution 34 is pumpedthrough the second electrode 122 b (after draining), which reduces,dissolves, and recaptures any solid S⁰. The polysulfide electrolytesolution 34 is passed in a first direction 80 a and through abi-directional filter 80 that is in a first auxiliary loop 82 a tocapture any residual Mn that precipitates. Similarly, but at a differenttime, the manganate electrolyte solution 136 is pumped through firstelectrode 122 a (after draining), which oxidizes and dissolves anyMn(OH)₂ precipitate. The manganate electrolyte solution 136 is passed ina second direction 80 b through the same bi-directional filter 80 but asa part of a second auxiliary loop 82 b to capture any residual S thatprecipitates. The bi-directional filter 80 enables recapture ofprecipitated species that are filtered out to be recaptured in thepolysulfide electrolyte solution 34 and the manganate electrolytesolution 136.

If the solids that result from crossover collect in the tanks 30 or 32,then these solids could be periodically removed from bottom reservoirsof the tanks 30 or 32 (the solids have significantly higher densitiesthan the liquids and thus sink). It is expected that this process wouldnot need to be done often, if at all, and does not need to be fullyautomated (i.e., this could be part of annual maintenance procedures).

Disproportionation reactions are a possibility, since Mn has a largenumber of oxidation states. If manganate disproportionates to Mn(V)O₄³⁻, the compound rapidly decomposes and precipitates to MnO₂, but understrongly alkaline conditions, this reaction is not a concern (i.e.,pH≥14). However, at high concentrations of NaOH, the following reactioncan occur slowly:4NaMnO₄+4NaOH→4Na₂MnO₄+2H₂O+O₂

Further reduction of manganate(VI) does not occur. The reaction is slow;measurements with a 4 M solution of MnO₄ ⁻ in 7.5M OH⁻ indicate acapacity retention of 80% after 1 month of storage of a fully chargedsolution. Nevertheless, this reaction will result in a permanentcapacity loss, unless a mitigation strategy, such as one describedbelow, is employed. Oxygen generation is also a concern since thereversible potential of the manganese couple is 157 mV higher than E^(o)for O₂ evolution (0.401 V vs. SHE) at pH=14. Therefore, the positiveelectrode material must be chosen to minimize catalyzing of O₂evolution. H₂ evolution is not a concern because the reversiblepotential for polysulfide is above E^(o) for H₂ evolution.

Small amounts of O₂ evolved from the disproportionation reaction, orproduced by the oxygen-evolution reaction in the positive electrode, canlead to electrolyte imbalance and result in energy capacity fade in theRFB. In this case, O₂ can be consumed by allowing it to react with thenegolyte by connecting the gas space above the posolyte and negolytereservoirs (this gas space shall be maintained as a N₂ blanket toprevent discharge of the anolyte):O₂+2H₂O+4Na₂S₂↔2Na₂S₄+4NaOH

The net of this reaction and the one above is a discharge of bothelectrolytes, but it results in the electrolytes being maintained at aconstant composition. Another result of these reactions will be anincrease of the pH of polysulfide electrolyte solution 34 and a decreasein the manganate electrolyte solution 136, but changes in waterconcentrations and [OH⁻] should be offset by diffusion through themembrane. If this is not the case, one can optionally utilize the pHadjustment cells and process described above for the Fe and S in method60, except only using the second step 64 to adjust for the decompositionof the manganese electrolyte (i.e., introduce O₂ gas into thepolysulfide, which is already included therein).

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthis disclosure. The scope of legal protection given to this disclosurecan only be determined by studying the following claims.

What is claimed is:
 1. A method for a redox flow battery, the methodcomprising: using first and second cells of a redox flow battery tostore input electrical energy upon charging and discharge the storedelectrical energy upon discharging, wherein each said cell has aseparator layer arranged between first and second electrodes, whereinthe using includes circulating a polysulfide electrolyte solution of pH11.5 or greater through a first circulation loop in fluid connectionwith the first electrode of the first cell, circulating an ironelectrolyte solution of pH 3 or less through a second circulation loopin fluid connection with the second electrode of the second cell, andcirculating an intermediator electrolyte solution through a thirdcirculation loop in fluid connection with the second electrode of thefirst cell and the first electrode of the second cell, and sulfur fromthe polysulfide electrolyte solution in the first electrode of the firstcell permeates through the ion-exchange layer of the first cell andprecipitates as a solid sulfide product in the second electrode and ironfrom the iron electrolyte solution in the second electrode of the secondcell permeates through the ion-exchange layer of the second cell andprecipitates as solid iron product; emptying the intermediatorelectrolyte solution from either the second electrode of the first cellor the first electrode of the second cell; and recovering either thesolid sulfide product to the polysulfide electrolyte solution or thesolid iron product to the iron electrolyte solution by, respectively,circulating at least a portion of the polysulfide electrolyte solutionfrom the first circulation loop through the second electrode todissolve, and thereby remove, the solid sulfide product from the secondelectrode of the first cell, and then transferring the polysulfideelectrolyte solution with the dissolved solid sulfide product back in tothe first loop, or circulating at least a portion of the ironelectrolyte solution from the second circulation loop through the firstelectrode to dissolve, and thereby remove, the solid iron product fromthe first electrode of the second cell, and then transferring the ironelectrolyte solution with the dissolved solid iron product back in tothe second loop.
 2. The method as recited in claim 1, includingmaintaining the intermediator electrolyte solution at a pH 12 or greaterso that the iron precipitates upon permeation through the ion-exchangelayer from the second electrode of the second cell into the firstelectrode of the second cell.
 3. The method as recited in claim 1,including maintaining the intermediator electrolyte solution at a pH 12or greater so that the sulfur precipitates upon permeation through theion-exchange layer from the first electrode of the first cell into thesecond electrode of the first cell.
 4. A method for a redox flowbattery, the method comprising: using first and second cells of a redoxflow battery to store input electrical energy upon charging anddischarge the stored electrical energy upon discharging, wherein eachsaid cell has a separator layer arranged between first and secondelectrodes, wherein the using includes circulating a polysulfideelectrolyte solution of pH 11.5 or greater through a first circulationloop in fluid connection with the first electrode of the first cell,circulating an iron electrolyte solution of pH 3 or less through asecond circulation loop in fluid connection with the second electrode ofthe second cell, and circulating an intermediator electrolyte solutionthrough a third circulation loop in fluid connection with the secondelectrode of the first cell and the first electrode of the second cell,using a third cell to electrolyze either the polysulfide electrolytesolution to produce hydrogen gas or the iron electrolyte solution toproduce oxygen gas; and maintaining the pH of polysulfide electrolytesolution to be pH 11.5 or greater or the pH of the iron electrolytesolution to be pH 3 or less by, respectively, introducing the oxygen gasinto the polysulfide electrolyte solution to adjust the pH of thepolysulfide electrolyte solution, or introducing the hydrogen gas intothe iron electrolyte solution to adjust the pH of the iron electrolytesolution.
 5. The method as recited in claim 4, wherein the introducingof the oxygen gas includes sparging the oxygen gas through thepolysulfide electrolyte solution.
 6. The method as recited in claim 4,wherein the introducing of the hydrogen gas includes sparging thehydrogen gas through the iron electrolyte solution.
 7. A method for aredox flow battery, the method comprising: using a cell of a redox flowbattery to store input electrical energy upon charging and discharge thestored electrical energy upon discharging, wherein the cell has aseparator layer arranged between first and second electrodes, whereinthe using includes circulating a polysulfide electrolyte solution of pH11.5 or greater through a first circulation loop in fluid connectionwith the first electrode of the cell, circulating an manganateelectrolyte solution through a second circulation loop in fluidconnection with the second electrode of the cell, and sulfur from thepolysulfide electrolyte solution in the first electrode permeatesthrough the ion-exchange layer and precipitates as a solid sulfideproduct in the second electrode and manganese from the manganateelectrolyte solution in the permeates through the ion-exchange layer ofthe second cell and precipitates as solid iron product in the firstelectrode; recovering either the solid sulfide product to thepolysulfide electrolyte solution or the solid manganese product to themanganate electrolyte solution by, respectively, circulating at least aportion of the polysulfide electrolyte solution from the firstcirculation loop through the second electrode to dissolve, and therebyremove, the solid sulfide product from the second electrode, and thentransferring the polysulfide electrolyte solution with the dissolvedsolid sulfide product back in to the first loop, or circulating at leasta portion of the manganate electrolyte solution from the secondcirculation loop through the first electrode to dissolve, and therebyremove, the solid manganese product from the first electrode, and thentransferring the manganese electrolyte solution with the dissolved solidiron product back in to the second loop.
 8. The method as recited inclaim 7, further comprising passing the polysulfide electrolyte solutionwith the dissolved solid sulfide product in a first direction through abi-directional filter and passing the manganate electrolyte solutionwith the dissolved solid manganese product in a second, oppositedirection through the bi-directional filter.